1. Field of the Invention
The present invention relates to a block noise removal device that removes block noise generated in decoding information data that are compression coded for each of blocks of the information data.
2. Description of the Related Art
Compression coding that employs a MPEG (Moving Picture Expert Group) method is often used for reducing the volume of information when video or audio signals are transmitted and/or recorded. With the MPEG coding processing, discrete cosine transformation (referred to hereinbelow as DCT) is implemented for each two-dimensional unit block with respect to a video signal, so as to obtain DCT coefficients of each frequency region, and then the volume of information is compressed by performing quantization processing. The larger is the quantization step used for the quantization processing, the higher is the compression ratio. However, because some values are omitted, the quantization noise is generated. Block noise is a typical example of such quantization noise. In the MPEG coding processing, because each of two-dimensional unit blocks is subjected to various processing, block boundaries appear when the blocks are decoded. Brightness signals and color difference signals are usually compressed when video signals are compressed. A variety of signal formats such as RGB signals can be considered as the video signal format.
Block noise removal devices that remove such block noise from MPEG-decoded video signals are known in the art. One example is disclosed in Japanese Patent Application Kokai (Laid-open) No. 2000-50275. This block noise removal device will be described with reference to FIG. 1 of the accompanying drawings. In this block noise removal device, the position of block boundary BB is detected from a video signal, and smoothing processing designed to correct the signal level is performed only to the pixels of the block boundary portion. As a result, the level transition in the block boundary portion is smoothed and the noise is removed.
However, such smoothing processing cannot achieve the effective noise removal when the difference in level between the adjacent blocks (BLOCK I and BLOCK II) is large.